This invention relates to roof trusses used in the construction industry to frame residential and light commercial buildings. More particularly, this invention is directed to the chord sections that are used to assemble roof trusses used in lightweight steel frame construction.
Wood is the predominant framing material used in residential and light commercial construction in the United States, However, builders, plagued by volatile and rising wood prices and poor quality as timber supplies shrink, continue to seek alternatives. Recent studies have identified steel as a promising alternative framing material to wood.
Various attempts have been made in the past to introduce lightweight, non-wood framing materials into the marketplace. These attempts include advanced composite materials such as fiber-reinforced plastic, as well as lightweight steel components such as doors, windows, siding and framing. However, history shows that whenever a new material becomes available to the construction industry, it is adopted cautiously, initially in small scale applications. Therefore, many of the newer wood substitute materials are not yet in wide use within the building industry. For example, in the instance of residential steel framing, acceptance has been slow because many builders have attempted to assemble lightweight steel framing using traditional wood construction techniques. Such wood construction methods drive up labor costs when applied to steel frame construction and make steel framing non-competitive with conventional wood frame construction. As a result, steel frame construction has gained only a small share of the home building marketplace as compared to wood frame homes. Steel frame construction tends to be concentrated mainly in areas where homes need to meet stricter structural demands to withstand natural phenomena such as earthquakes, high hurricane force winds, and pest problems such as termites.
However, with the adoption of new building techniques that include, for example, prefabricated steel frame panels delivered assembled to the construction site, and with the availability of new screw guns and fasteners that facilitate and improve steel frame connections, residential steel framing is gaining in popularity within the building industry. In particular, residential roof framing is one area that currently offers improved opportunities for using wood substitute construction materials. Manufactures have introduced an array of different non-wood roof framing products that range from steel roof panels, rafters and purlins, to prefabricated lightweight steel frame roof trusses designed to carry heavy loads over long spans.
The state-of-the-art for non-wood roof truss designs, is dynamic. Numerous different steel truss design improvements have taken place over a relatively short period of time, with many of these improvements directed to the shape of the structural sections used for the top and bottom chord members of the truss. It has been discovered, however, that, past steel truss chord sections present a plethora of problems for roof truss fabricator as well as for home builders.
For example, in FIGS. 6 and 13 of U.S. Pat. No. 4,435,940 to Davenport, et al., FIGS. 2 and 5 of U.S. Pat. No. 4,982,545 to Stromback, and in FIGS. 3 and 6 of U.S. Pat. No. 4,986,051 to Meyer, roof truss chord sections are shown comprising outward extending flanges. Such outward extending flanges stiffen and improve the strength of truss chords. However, outward extending flanges prevent the chords from lying flat during shipping and handling, and make it awkward to manufacture the roof truss. Additionally, outward extending flanges expose sharp sheet metal edges, and workers handling such chord sections must exercise extreme caution to avoid serious cuts, lacerations and other injuries.
U.S. Pat. No. 5,463,837 to Dry, teaches forming an outside hemmed edge along both legs of a truss chord. This would tend to protect workers from injury. The radiused hem edge eliminates the sharp edges associated with the outward extending flanges taught in the above three earlier patents. However, tests show that such hemmed edges greatly reduce the roof truss chord section properties to undesirable levels when compared to the outward extending flanges cited above.
Other lightweight steel frame sections teach providing an inward extending flange that maintains good section properties. For example, FIGS. 1, 3, 5, 7 and 9 of Meyer's U.S. Pat. No. 5,157,883, shows inward extending flanges. The 883 Meyer patent is directed to vertical studs used in lightweight steel framing. Another example of inward extending flanges, in a roof truss, is shown in FIGS. 4 and 7 of U.S. Pat. No. 4,982,545 granted to Strombach. While such inward extending flange sections would tend to reduce worker injury, maintain good section properties, and allow the sections to lie flat during roof truss fabrication, they create a new set of problems for the truss manufacturer.
A typical roof truss comprise a plurality of web members that extend between the top and bottom chord members of the truss. Each web member is inserted between the legs of the top chord and between the legs of the bottom chord member, and each truss web member is fastened to the chord members using self-drilling sheet metal screws that extend through the chord legs and into the web members or struts. In instances where the truss chord sections include inward extending flanges, prior to the present invention, it has been impossible to use self drilling screws or other simple fasteners to make the necessary truss chord-to-web connections. As clearly shown in the Meyer patent, the inward extending flanges create a large gap or space between the chord legs and inserted web member. Special connection hardware must be used to fasten the truss web members to the top and bottom truss chord members, as illustrated in FIG. 9 of Meyer, and such hardware is expensive to produce and time consuming to use.
In an attempt to overcome the aforementioned problems, one truss builder is manufacturing and selling a truss chord section that has inside hems formed along the top edge of both chord legs. The hems are formed with a tight radius in order to be coplanar with a corresponding leg surface that engages the truss web members that are inserted between the legs of the chord section. This roof truss design allows the truss chords to lie flat during roof truss fabrication, eliminates sharp sheet metal edges along the chord legs, and enables fabricators to make truss chord-to-web connections using self-drilling sheet metal screws. However, as stated above for the outside hems, tests show that hemmed edges produce very undesirable section properties in the truss chords. Additionally, in cases where the inside hems become deformed, whether during forming operations or during shipping and handling, deformed hems interfere with inserting the truss web members into the chord sections during fabrication of the roof truss. The chord legs must be pried apart to provide clearance between deformed hems, and this produces a gap between the truss web member and the chord leg that causes the self-drilling screws to fail to seat properly when the truss chord-to-web connections are made. Such defective connections are rejected if discovered during product quality inspection, or may fail prematurely if used under actual loading conditions.